Java default methods are not overridden by delegation

Reports delegation when the delegate object is an instance of a class that overrides (in the case of final and sealed classes) or may override (in the case of open and abstract classes) Java default methods.

By design, Kotlin delegation does not override Java default methods, which can lead to subtle bugs, especially in cases of deep inheritance hierarchies.

Example:

// Base.java:
public interface Base {
  default void print() {
    System.out.println("Base");
  }
}

// BaseImpl.kt:
class BaseImpl : Base {
  override fun print() {
    println("BaseImpl")
  }
}

// Main.kt:
fun main() {
  val inherited = BaseImpl()
  val delegated = object : Base by inherited {}
  inherited.print() // Outputs: BaseImpl
  delegated.print() // Outputs: Base
}

Developers typically expect that all methods, including default methods overridden in the delegate, are forwarded. Instead, Java default methods are called, which can lead to unexpected results.

One quick-fix overrides Java default methods by delegating their implementation to the delegate object:

// Main.kt:
fun main() {
  val inherited = BaseImpl()
  val delegated = object : Base by inherited {
    override fun print() {
      inherited.print()
    }
  }
  inherited.print() // Outputs: BaseImpl
  delegated.print() // Outputs: BaseImpl
}

Another quick-fix overrides Java default methods by explicitly delegating their implementation to the superclass:

// Main.kt:
fun main() {
  val inherited = BaseImpl()
  val delegated = object : Base by inherited {
    override fun print() {
      super.print()
    }
  }
  inherited.print() // Outputs: BaseImpl
  delegated.print() // Outputs: Base
}

Locating this inspection

By ID

Can be used to locate inspection in e.g. Qodana configuration files, where you can quickly enable or disable it, or adjust its settings.

JavaDefaultMethodsNotOverriddenByDelegation

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Path to the inspection settings via IntelliJ Platform IDE Settings dialog, when you need to adjust inspection settings directly from your IDE.

Settings or Preferences | Editor | Inspections | Kotlin | Java interop issues

Inspection ID: JavaDefaultMethodsNotOverriddenByDelegation

New in 2025.1

Suppressing Inspection

You can suppress this inspection by placing the following comment marker before the code fragment where you no longer want messages from this inspection to appear:

//noinspection JavaDefaultMethodsNotOverriddenByDelegation

More detailed instructions as well as other ways and options that you have can be found in the product documentation:

IntelliJ IDEA 2025.3

Inspection Details
By default bundled with:	IntelliJ IDEA 2025.3, Qodana for JVM 2025.3,

Last modified: 03 December 2025